Between the tip end of a rotor blade of a gas turbine and a shroud facing the tip end of the rotor blade, a clearance of a predetermined size is provided to prevent the tip end of the rotor blade from coming into contact with the shroud during the operation. If the clearance is too large, combustion gas leaks from the pressure surface side to the negative pressure surface side of the rotor blade, so that a pressure loss increases, and thus the operation efficiency of gas turbine decreases. In order to prevent this phenomenon and to thereby improve the performance of gas turbine, attempts have been made to set the clearance at the minimum.
If the clearance is too small, at the early stage of operation start of gas turbine, the tip end of rotor blade and the shroud slide on each other due to thermal expansion of rotor blade, eccentricity of turbine rotor, vibrations occurring in the whole of gas turbine, etc. (what is called initial sliding). Also, if the gas turbine is operated for a long period of time, the shroud exposed to a high-temperature gas is gradually subjected to thermal deformation, which sometimes causes sliding of the tip end of rotor blade and the shroud (what is called secondary sliding). Heavy sliding of the tip end of rotor blade and the shroud occurs at the time of initial sliding. Contrarily, the secondary sliding is relatively moderate.
Generally, the shroud is provided with a coating layer, which is formed to shut off heat or prevent oxidation, on the inner peripheral surface thereof. For example, for the purpose of heat insulation, a TBC (Thermal Barrier Coating) is provided, or an oxidation-resistant coating formed of M-Cr—Al—Y (hereinafter referred to as MCrAlY) is provided. In many cases, these coatings have a high hardness. Therefore, if the tip end of rotor blade slides on the inner peripheral surface of shroud, the rotor blade may be damaged greatly.
Japanese Patent Provisional Publication No. 4-218698 (No. 218698/1992), National Publication of International Patent Application No. 9-504340 (No. 504340/1997), Japanese Patent Provisional Publication No. 10-30403 (No. 30403/1998), and U.S. Pat. No. 5,702,574 have disclosed a rotor blade having an abrasive layer in which abrasive particles are dispersed in a matrix consisting of MCrAlY, which is an oxidation inhibiting material. For this rotor blade, as the abrasive particles, for example, cubic boron nitride (CBN) is used. The cubic boron nitride is a hard material. Therefore, if the tip end of rotor blade slides on the inner peripheral surface of shroud, the abrasive particles consisting of cubic boron nitride grinds the inner peripheral surface of shroud. Thereby, a proper clearance is kept between the rotor blade and the shroud.
However, since the cubic boron nitride has insufficient heat resistance, it is turned into boron oxide and sublimated by a long-term operation in an oxidizing atmosphere, so that the abrasive particles sometimes disappear. Also, the abrasive particles sometimes fall off gradually as MCrAlY is deteriorated by oxidation. Therefore, at the time of initial sliding, the shroud is ground by the abrasive particles. However, in the secondary sliding after the abrasive layer has been deteriorated, the grinding is insufficient, so that the rotor blade may be damaged greatly. In particular, in recent years, a tendency for the combustion temperature in a gas turbine to increase from the viewpoint of the improvement in energy efficiency has been found, which makes a problem of heat deterioration in abrasive layer remarkable. The same problem can be found in a combustion engine such as a jet engine.